Advances in polymerization and catalysis have produced new polymers having improved physical and mechanical properties useful in a wide variety of products and applications. High density polyethylene compositions, for example, are known to be useful for making a variety of commercial products such as films, pipes, and blow molding products. In particular, “bimodal” or “multimodal” high density polyethylenes (bHDPE) are useful in this regard.
In blow molding applications the polyethylene melt flow ratio (MFR) is an important parameter in achieving a good balance of properties. In extrusion blow molding (EBM) applications, bottle weight (weight swell) defined as the post extrusion swelling of the resin as measured by the weight of a bottle blown from the polymer resin, is a critical variable.
Bimodal high density polyethylenes may be produced in a dual reactor system using traditional Ziegler-Natta catalysts. Generally these bimodal resins have a relatively low weight swell. In contrast, unimodal high density polyethylenes produced by a chromium catalyst (Phillips catalyst), generally have a high weight swell. Accordingly, for a given polyethylene production unit, switching between low and high swell polyethylene resins may require switching between quite different catalyst types and reactor configurations. Clearly, this is undesirable and adds complexity to the production process. It would therefore be advantageous to provide a process that overcomes these disadvantages and which may provide access to both high and low swell polyethylene resins.